a) Field of the Invention
The invention is directed to an arrangement for generating pulsed currents for gas discharge pumped radiation sources, particularly with high current strengths (e.g., 10 to 50 kA at voltages of several kV). The electrical pulses with high repetition rates (preferably 1 Hz to more than 5 kHz with energies of 5 to 20 J) serve to generate a hot plasma which emits light in the EUV range. The invention is advantageously applicable in all electrical discharge methods for plasma generation such as Z pinch, hollow cathode, capillary and plasma focus discharges.
b) Description of the Related Art
In the semiconductor industry, photolithography methods in which a scanner contains a mask (with the structure to be imaged) that is imaged in a reduced manner (typically 1:5) on the wafer are currently applied for the fabrication of microchips. In addition to special lamps, narrow-band excimer lasers with wavelengths of 248 nm and 193 nm are principally used as radiation sources for exposure. Scanners based on F2 lasers (157 nm) are in development.
For the next generation of lithography machines, exposure with EUV sources at wavelengths around 13.5 nm appears to be the most promising variant for generating even smaller structures on the wafer. It is known for this purpose to use a plasma as the source of the EUV radiation. This plasma can be generated by means of focused laser radiation or by an electrical discharge.
A gas discharge pumped Z pinch plasma pertains to a gas discharge with cylindrical geometry in which the plasma is generated by a high current flow and by magnetic compression is pinched to form a thin thread of about 1 mm. Average plasma energies of greater than 30 eV are achieved.
In order to achieve a high sample throughput in chip fabrication with EUV lithography, a power of several hundred watts is required. This requires EUV radiation pulses of about 10 mJ per detected solid angle at repetition rates of greater than 5 kHz.
Apart from the characteristics of the optical system (numerical aperture, depth of focus, aberrations or imaging errors of the lenses or mirrors) and the structure of the resist material, the image quality of photolithographic methods is essentially determined by how accurately the radiated radiation dose can be maintained. This dose stability, or dose accuracy, is determined by:
a) pulse quantization;
b) pulse-to-pulse stability; and
c) spatial stability of the emitting volume.
Pulse quantization (a) is scanner-specific, since the quantity of light pulses that can fall into the exposure gap (moving slit) during an exposure process (scan) can vary. However, this factor can usually be ignored.
Contributing factors b and c are specific to the radiation sources themselves. Factor c is significant only for EUV sources.
The requirements of chip manufacturers with respect to dose stability at the site of the wafer set extremely high demands on pulse-to-pulse stability. This is expressed as a standard deviation σ of the actual light pulse energy from the average of the light pulse energy or from the targeted pulse energy (set energy value). In DUV lithography and VUV lithography, σ-values of less than 1.5% are permitted for narrow-band excimer lasers, whereas in EUV lithography σ-values of less than 0.4% are required. These limits can be adhered to only by means of special energy regulation; even when unregulated, the σ-values must be less than 3%.
All of these demands, high EUV output and low σ-values, translate to high requirements on the electric power supply for an EUV radiation source of this type.
A high-power pulsed current supply is known from U.S. patent application 2002/01633133 A1, which discloses a circuit for pulsed generation of EUV sources and x-ray sources for generating electrical pulses of at least 12 J at a repetition rate of at least 2 kHz. The circuit arrangement contains a capacitor bank, a fast resonant charging circuit which charges the capacitor bank, a control circuit which monitors the charge voltage, and a trigger circuit by which the capacitor bank is discharged in a magnetic compression circuit in order to generate light pulses in the EUV or x-ray range with a time constant of less than 10 ns.
This solution is disadvantageous in that a powerful transformer (usually oil-cooled) is required for generating the high discharge voltage and the resetting of the magnetic compression circuit must be carried out by an elaborate external bias circuit. Further, as a result of the relatively low high voltage (up to about 1.3 kV), the total capacitance of the storage banks must be greater than 24 μF with the given stored energy (approximately 20 J) and the high voltage can only be recharged in a controlled manner by means of a plurality of switches and a large inductor. The charging voltage stability is accordingly dependent upon the resonant charging circuit and the attainable repetition frequency has a ceiling imposed by the time constant of the resonant recharging process.